In the above equation, \(DM_{\rm MW}\)  is the contribution of the interstellar medium in the Milky Way, which can be derived from the NE2001 Galactic free electron density model [1]} or the YMW model [2]}. \(DM_{\mathrm {halo}}\)  is contributed by the free electrons in the Galactic halo. [3]} found that the \(DM_{\mathrm {halo}}\)  is 50 pc cm\(^{-3}\)  < \(DM_{\mathrm {halo}}\)  < 80 pc cm\(^{-3}\) . Recently, [4]} estimated that the mean \(DM_{\rm halo}\)  is 43 pc cm\(^{-3}\) , with a full range of 30-245 pc cm\(^{-3}\) . [5]} derived host contribution \(DM_{\rm host}\)  distributions of repeating and non-repeating FRBs with the IllustrisTNG simulation. They found that the distributions of \(DM_{\mathrm {host}}\)  can be well fitted by a log-normal function. For non-repeating FRBs, the median of \(DM_{\mathrm {host}}\)  is about \(30 - 70\)  pc cm\(^{-3}\)  in the redshift range \(z=0.1-1.5\) . The \(DM_{\mathrm {source}}\)  depends on the central engine of FRBs. If FRB is generated by mergers of binary neutron stars [6]}, [7]}, the value of \(DM_{\mathrm {source}}\)  is small [8]}, [9]}.
